1. Field of the Invention
The invention concerns the field of electrophysiology, and especially the detection by electrodes, in a difficult medium and in order to record them, of electrophysiological signals of the electromagnetic type, emitted by a subject.
2. Description of the Related Art
It concerns more particularly the devices of the type comprising sensor equipment that can be fitted to a subject, and provided with a set of electrodes for detecting electromagnetic signals, in order to deliver them in the form of output signals to inputs of processing means capable, moreover, of processing them prior to storage.
Such a set generally comprises working electrodes having at least one acquisition electrode installed in a selected position in or on the sensor equipment, and also an additional electrode to be connected to the processing means as a floating ground.
By sensor equipment there is to be understood, for example, a helmet provided with electrodes intended to detect cerebral activity, of the electroencephalogram type (EEG), or an apparatus intended to detect muscular activity, of the electromyogram type (EMG), or an apparatus intended to detect cardiac activity, of the electrocardiogram type (ECG), or more generally any apparatus for the acquisition of electrophysiological signals.
Furthermore, by subject there is intended to be understood any living being (human or animal), whether clinically ill, and in this case it is a patient, or clinically healthy.
The Applicant observed that, in certain conditions of use, the signals detected by the electrodes of such sensor equipment exhibited a signal/noise ratio (S/N) which was of the order of unity, and consequently did not really represent the activity under examination.
This is particularly the case with examinations intended to detect electrical signals, on the one hand, of very small amplitude, typically of the order of a few microvolts, on subjects that are subjected to imposed displacements, such as, for example, during stimulation of their vestibular system (in order to gather vestibular evoked potentials), and/or, on the other hand, on subjects which are not easy to keep immobile, such as, for example, babies, hyperkinetic subjects or patients suffering from Parkinson's disease.
This is also the case when making recordings in environments which are noisy from an electromagnetic point of view, even when the subject is immobile.
The Applicant observed that the principal cause of this effect of noise interference on the signals could be attributed to the coupled effect of the impedance of the electrodes (typically a few kilo-ohms) and of the electrical loops which are formed by the connecting cables between the electrodes and the processing means. These loops, when they are subjected to displacement in a continuous electromagnetic field, have at their terminals an induced interference voltage comparable to electrical noise.
Such a voltage is so much the more disturbing when, in the case of imposed displacements, it is in phase with the signals representing the activity under examination, and the larger the size of the loop and the greater the impedance of the loop, and consequently of the electrodes, the greater the amplitude of the voltage is.